This invention is related generally to the art of the construction and operation of lasers, and, more specifically, to the structure of a laser resonating cavity utilizing spatial filtering.
The basic structure common to nearly all lasers is well known. A light amplifying medium is positioned between two mirrors that forms a cavity between them that is resonant to oscillations of light having a wavelength desired to be produced. An output beam of the laser can be derived through one of the mirrors, in which case it is made to be only partially reflective, or around one of the mirrors, in which case it is made totally reflective. Alternatively, an output can be obtained by use of a beam splitter in the cavity or by cavity dumping. The light amplifying material may be in a gaseous, liquid or solid form, depending upon the type of laser and its specific desired characteristics. Lasers are also made to either operate with a continuous wave (c.w.) light output, or are designed to provide repetitive high energy light pulses, the structure of each of these types of lasers being quite different.
Laser oscillator resonators are generally classified into one of two basic types. A "stable" resonator has its end mirrors shaped to confine the beam within the resonant cavity. Oscillation is generally limited to the fundamental, transverse mode (TEM.sub.00) by the use of an appropriate aperture within the cavity. The laser output beam has a smooth Gaussian intensity profile across it, a very desirable characteristic. However, the controlled size of the laser beam within the oscillator is very small unless either the resonant cavity is made to be much longer than usually desired or other optical elements are used in the cavity with undesirable side effects. A small beam necessarily interacts with only a limited volume of the light amplifying material. The result is a laser output beam with limited energy. However, its intensity profile is a Gaussian function, so stable resonators are often used where a high energy output is not so important.
The second general type of laser oscillator resonator is referred to as an "unstable" type. In this type, the beam is not confined within the cavity but escapes after a limited number of round trips, contrary to the stable resonator. The cross-sectional size of the beam increases during each round trip through the resonator. Such a resonator has an advantage of developing a higher energy output beam because it interacts with a full volume of the laser rod. However, it has the disadvantage of providing a laser output beam with a poor intensity distribution across it because of the edge diffraction effects of an abrupt aperture with which the expanding beam necessarily interacts.
Whether a given laser resonating cavity is defined as a stable or unstable type depends entirely upon dimensions of the geometric optics within the resonator. Specifically, the radii of curvature of the end mirrors and the resonator cavity optical length between mirrors determine whether it operates as a stable or unstable type. A mathematical treatment of this is set forth in a reference book by Professor Anthony E. Siegman, Lasers. University Science Books, Mill Valley, Calif., 1986, specifically at pages 561-562 and 744-749.
A great deal of effort is being directed toward the development of a resonant cavity that combines the advantages of the stable and unstable types while leaving their disadvantages behind. Particularly, it is desired to obtain the high power output of the unstable resonator with the beam quality of the stable resonator. One approach that has been pursued in various configurations is to use an unstable resonator type, for its high power and efficient use of the amplifier medium, but with inclusion of spatial filtering within the resonator to improve the quality of its output beam, primarily the intensity distribution across it. One of these configurations is a negative branch confocal, self-filtering unstable resonator, commonly referenced as a "SFUR". This configuration is described, for example, by Gobbi et al., Optics Communications, Volume 52, No. 3, 1 December 1984, pages 195-198, and in U.S. Pat. No. 4,787,092 --Gobbi et al. (1988). In this configuration, one of the end mirrors causes the light in the resonator to come to a focus at a small pinhole in a spatial filter arrangement. Although this configuration has been recognized as a significant advance, it suffers from a disadvantage that precise alignment is required between the pinhole spatial filter and the spot focus of the end mirror. This can be difficult to maintain in a commercial product.
Therefore, it is a primary object of the present invention to provide a laser resonator structure which overcomes this disadvantage.
It is a more general object of the present invention to provide a laser resonator structure that efficiently utilizes the amplifying media therein but which, at the same time, provides an output beam having a very smooth intensity distribution across it.
It is another object of the present invention to provide a laser resonator that is simple in construction, reliable and easy to operate, and which permits a wide range of specific design parameters without affecting its advantageous operation.